Rotating seals used in tools and equipment which operate in high hydrostatic pressure environments require a pressure equalizing element. For example, rotary rock bits having an o-ring packing seal between each rotary cutter and journal pin thereof typically include a lubrication system. The system includes a reservoir filled with a lubricant, passages to communicate the reservoir with the bearing area between each rotary cutter and journal pin, and a flexible diaphragm pressure compensator. The diaphragm is positioned within the reservoir to limit the pressure differential between the lubricant and the pressure in the well bore. In operation, a portion of the flexible diaphragm moves axially within the reservoir to equalize the pressure differential between the lubricant reservoir pressure and the bore hole fluid pressure, therefore minimizing the pressure differential across the o-ring seal. Such lubrication systems often advantageously include a high pressure relief valve to relieve the excess lubricant pressure developed during the heating and/or raising and lowering of the drill bit.
Lubrication systems incorporating axially-expanding diaphragms have proven generally effective for equalizing the pressure across a rotating seal. However, as seen in U.S. Pat. No. 4,276,946 to Millsaps, Jr., such diaphragms are typically cup-shaped and thus include folded sidewalls. Because of this diaphragm structure, an additional force, i.e., the force required to "roll" these sidewalls, must also be overcome before the bottom of the diaphragm can move axially. Moreover, such cup-shaped diaphragms may not operate properly if the cup moves offcenter with respect to the longitudinal axis of the reservoir. In such systems, the lubricant differential pressure is often greater than 20 psi during normal operating conditions. Most types of o-ring packing seals perform best with small pressure differentials, i.e., on the order of 10 psi. Seals which run at minimal pressure differentials have greater effective lives when required to withstand high surface velocities or higher ambient temperatures which are encountered in deep drilling. Moreover, such cup-shaped diaphragms may not operate properly if the cup moves off-center with respect to the longitudinal axis of the reservoir. Therefore, there is a need for an improved diaphragm structure which is free operating and has no inherent restrictions which require increased pressure to overcome.